Synthetic fibers with increased surface friction



April 27, 1965 H THOMPSQN 3,180,785

SYNTHETIC FIBERS WITH INCREASED SURFACE FRICTION Filed March 21, 1962United States Patent 3,180,785 SYNTHETIC FIBERS WITH INCREASED SURFACEFRICTION Hugh Brian Thompson, Milton, Mass., assignor to The KendallCompany, Boston, Mass., a corporation of Massachusetts Filed Mar. 21,1962, Ser. No. 181,354 4 Claims. (Cl. 161-169) This invention relates toa method for enhancing the strength of fibrous arrays comprising certainsynthet1c fibers, and the product thereof.

More particularly it is concerned with a method for substantiallyincreasing the strength of non-woven fabrics prepared from certainpolymeric fibers, said fabr cs bemg primarily dependent for theirenhanced integrity upon frictional engagement of the fibers.

Non-woven fabrics produced from synthetic fibers, particularly of thepolyester type, are increasingly important articles of commerce,especially in the form of lightly felted batts or arrays which may becompressed to a feltlike consistency, usually with the accompaniment ofa certain degree of needling, punching, or other processing forentangling, bonding, or intermeshing the fibers into a unitary, coherentstructure. Non-woven fabrics of this type are finding increasing usagein the interlining field, as well as in electrical and other industrialfields.

The degree to which such non-woven fabrics of synthetic fibers may beutilized, however, is definitely limited by the low strength whichcharacterizes prior art non- Wovens of this type. In general, non-wovenfabrics derived from a given Weight of fiber are very much lower intensile strength than woven fabrics derived from the same weight ofstaple fiber. The process of spinning and twisting the fibers into yarnsbrings inter-fiber frictional forces into play, which greatly enhancethe strength of the fabric woven from such yarns. Additionally, in aconventional yarn structure, an applied load is distributedsimultaneously over a large number of fibers, whereas in a non-wovenfabric the cooperative contact between fibers is of a low order ofmagnitude, and the potential strength of the fibers is not realizedsince they react to stress almost singly and independently.

In an effort to enhance the strength of non-woven fabrics, variousexpedients have been resorted to, such (1) .Saturating fibrous battswith heavy concentrations of bonding agents: This not only adds foreignmaterial to the fibrous array, but has an adverse effect on thedesirable properties of softness and flexibility, which are oftenprimary requisites as in the interlining field.

(2) Needle-punching the fibrous batts by repeated passes through aneedle-loom: Each pass, however, makes the array more dense and compact,and is in addition uneconomical.

(3) Reinforcing the fibrous batts with yarns or fabrics: This is notonly expensive. but it decreases the pliability, conformability, andextensibility of the batt.

(4) Strengthening the batt by using thermoplastic binder fibers, or bymixing the synthetic fibers with naturally feltable fibers, oralternatively by employing certain retractable synthetic fibers in aretracting shrinkage process, as set forth in the patent to Lauterbach,US. 2,910,763: Various other proposals have been made wherein the fibershrinkage is accompanied by a certain amount of fusion and stickingtogether of thermosensitive fibers.

Such processes employing fusion and retraction, however, are difiicultto control insofar as they depend on the use of fibers which retractwhen heated. The retraction of the fibers inevitably results inshrinkage of the Whole assembly, and in a certain degree ofdensification Patented Apr. 27, 1965 whereby the soft appealing hand ofthe original fibrous array is to some extent lost or destroyed.

It is an object of this invention to provide a strong nonwoven felt-likearray of synthetic fibers which is not characterized by having undergoneshrinkage or fiber fusion. It is a further object of this invention toprovide a process for substantially increasing the strength of such afelt-like array without substantial retraction or fusion of thesynthetic fibers. It is also an object of this invention to provide aprocess for making a high tensile strength array of fibers which retainsessentially all of the softness, drape, and conformability of theunprocessed array.

It is a further object of this invention to provide an array ofsynthetic fibers characterized by a substantially increased degree ofinter-fiber friction, whereby the tensile strength of the array issimilarly increased.

These and other objects will be apparent from the following descriptionof the invention.

In general, the elements of this invention are based on considerationsof and means for enhancing the fiber-tofiber friction of fibrous arrays,without adding foreign substances or shrinking the fibers to the extentthat the density, compactness or conformability of the array becomeobjectionable. In this respect, my process is applicable to a fibrousarray, such as a needled batt of polyethylene terephthalate fibers, andoperates so as to impart to such a batt a 10 to 20 fold increase instrength without substantial shrinkage, and without surrendering to anysignificant degree the desirable properties of softness, flexibility,porosity, etc., associated with what I term fiber freedom.

It is known to enhance fiber-to-fiber fricton by the use of colloidalsilica or similar materials. However, the presence of added foreignmaterials is often undesirable, and their efficiency in promoting fiberfriction is limited, since they are merely in contact with the fibersurface.

I have found that unexpectedly large-scale increases in fiber-to-fiberfriction may be brought about by a heat treatment of certain syntheticpolymeric fibers, as typified by fibers made from polyethyleneterephthalate. Commercially available fibers of polyethyleneterephthalate, such as offered by Du Pont as Dacron 54, or by Celaneseas Fortrelle, contain a small amount of lower molecular weight polymericmaterial which appears to be principally a trimer of the ethyleneglycol-terephthalic acid condensation product, in cyclic form. Thistrimer may be associated with smaller amounts of other oligomers.

Normally, the presence of these lower molecular weight materials incommercial polyester fibers is not obvious or apparent even undermicroscopic examination at very high magnifications. The lower polymerapparently exists within the body of the fiber, which shows a smoothsurface. By exposing the fiber to a heating process, however, I havefound that I can cause to appear on the fiber surface'an irregularcrystalline growth which clings tenaciously to the fiber surface,converting the fiber profile from an originally relatively smooth,low-friction contour to one which is marked by rough, sharp-edgedprotuberances. This effect is illustrated in the drawing, wherein FIGURE1 is a representation of a portion of the length of a commercialsynthetic fiber. FIGURE 2 is a representation of the same portion offiber after treatment according to this invention, showing crystals 12,derived from the body of the fiber 10, the crystals being adherent toand lying on the surface of the fiber, and being in the general natureof sharp-edged flat prisms.

As set forth more fully below, this can increase the inter-fiberfriction to such an extent that a heat-treated needled felt made fromsuitable fibers may have a tensile a strength 20 times the strength ofan identical needled felt before heat treatment.

It is not known to me whether the lower molecular weight material, whichI will call trimer for convenience, exists in the form of crystalswithin the body of the unheated polyester fiber, or whether it isdisposed in crystallites, or in an amorphous, non-crystalline state ofaggregation.

In any case, it appears that the process of this invention yieldsproducts which are chemically and physically unique with respect tosurface roughness and fiber-to-fiber friction, so far as man-made fibersare concerned. The art of adding foreign crystalline material to fibers,to enhance friction, is known. It is also possible to bondfriction-promoting agents to fibers, either by adhesives, or by heatingthermosensitive fibers in the presence of such agents, or by swellingcertain sensitive fibers and adding foreign material while the fibersare surfacesoftened or surface-swollen. Several considerations make suchtreatment undesirable, however. No matter what sort of adhesive is used,there is always difficulty in promoting a maximum degree of adhesion offriction-enhancing agent to fiber, since the ideal conditions forspecific adhesion between two different chemical species are hard toestablish. Second, a foreign material, no matter how finely divided, canrarely be so dispersed that it is distributed over the fiber surfacewith maximum efficiency. The addition of foreign friction-enhancingagents from an external phase, onto the fiber surface, will alwaysremain a different process from the promotion, from within the fibersubstance, of crystalline growths which grow outwardly from the fibersurface in the formof rough protuberances. In the latter case, theprotuberances are of the same basic chemical composition as the mainbody of the fiber, so that questions of specific adhesion do not arise,nor is a foreign chemical identity added to the fiber system.

It should also be appreciated that the fiber-to-fiber frictionalenhancement of this invention differs from the results obtained byetching fiber surfaces. or partial-dissolution process, highly irregularareas are dug away or eaten away from the fiber surface. This is asubtractive effect, wasteful of fiber substance, and always accompaniedby a decided degree of fiber weakening, from the very nature of theprocess.

It will be appreciated that this invention is applicable to a widevariety of fibrous arrays including card webs, needled batts, bondednon-woven fabrics, roving, and other fibrous aggregates wherein thesurface friction of fiber against fiber constitutes one of the mainelements in the strength of the product. It is also applicable tofabrics made from low twist or zero twist yarns, which may bestrengthened for the weaving process by use of a sizing agent which maybe subsequently removed prior to or during the practice of my inventionon such fabrics. The increased strength shown by the process of thisinvention will vary with the geometry of the fibrous array in question.For example, in an unpressed card fleece, the fibers are normally inengagement only along a small fraction of their length, since thedensity of such material is very low, being only about 1% of the densityof the fiber substance. Under such circumstances, as illustrated inExample I, the fibers make little contact with each other, and thestrength increase shown by the practice of this invention, thoughuseful, is only about 100%.

Much higher strength increases, up to twentyfold, are shown by needledbatts of suitable fibers, as illustrated by Example II. To some extent,at least, this can be explained by the increased fiber-to-fiber contactshown by needled webs, so that the surface roughness introduced by theprocess of this invention has a greater chance to take effect.

As illustrated in Example III, the treatment of my invention producesonly moderate strength increases in In an etching fibrous arrayspreviously bonded by the use of thermoplastic fibers. This is notsurprising in view of the consideration that in such products, theprincipal source of strength is the bond between the thermoplasticfibers and the non-binder fibers, which is so high as to obscure theeffect of slippage between non-binder fibers. Such bonded webs are, ofcourse, much less flexible and conformable than needled batts of thesame composition.

Example I A quantity of 3 denier 1 /2 inch polyethylene terephthalatefiber of the type known commercially as Dacron 54 (E. I. du PontCompany) was carded on conventional textile carding equipment to form anassembly of superimposed webs weighing 183 grams per square yard. Toincrease the fiber-to-fiber contact of this rather fluffy assembly, theassembly of webs was passed through a cold textile calender at apressure of 200 pounds per inch of nip, to form a compressed batt.

Inch-wide strips were cut from this batt and measured in an Instrontensile tester using 3-inch jaws 3 inches apart moving at 5 inches perminute. The tensile strength of the strips averaged 0.332 pound.

A part of the rest of the batt was subjected to a temperature of 400 F.in an air-circulating oven for 3 minutes. There was less than 10%shrinkage in the batt, no appreciable change in softness and porosity,and no evidence of fiber fusion or retraction. In a tensile testparallel to that set forth above the heated batt showed a strength of0.697 pound, an increase of 110%.

Example II A quantity of the same fiber used in Example I was carded toform a batt which was passe-d through a needle loom at such a rate as toimpose on the batt about 370 penetrations per square inch, the needlingprocess being carried out on both faces of the batt. The needled fibrousarray weighed 100 grams per square yard.

Tensile strength of inch-wide strips of this material averaged 1.45pounds, as measured on an Instron tensile tester with 3-inch jaws 3inches apart, moving at 5 inches per minute.

A sample of this needled material was heated at 400 F. for 4 minutes ina circulating air oven. Comparable tensile tests on one-inch wide stripsof heated material showed a strength of 26.9 pounds, or between 18 and19 times the strength of the unheated needled batt.

Shrinkage was inconsequential, being less than 15%, and there was noevidence of fiber fusion, or of loss of softness or porosity due to heattreatment.

In a parallel set of tests, a similarly needled batt was heated to 250F. for 39 hours, and showed a strength increase of 8.8 fold. Anothersimilar batt heated to 300 F. for 3 hours showed a 12.6 fold increase instrength. For the realization of maximum strength in the shortest time,therefore, I prefer to operate in the neighborhood of 400 F. whenprocessing fibers such as Dacron 54. At temperatures much in excess of400 F., secondary effects of fiber softening and shrinkage begin tooccur, and the strength per unit weight falls off rather rapidly.Additionally, heating to above 400 F. for longer periods seems tosublime the surface crystal deposit off the fiber surface, as'

followed by microscopic examination and tests of tensile strength, whichdecreases under these conditions.

Although the strength increase shown by treatment at 400 F. for 4minutes is considerably more than that at 250 F. for 39 hours, this isprobably not directly attributable to the higher temperature havingcaused a higher amount of surface crystal growth. The nature of thegrowth is important: high temperatures result in numerous small crystalson the fiber surface, while the 250 F. treatment resulted in larger, butfewer, crystals, which might be expected to show a lower degree ofenhancement of fiberto-fiber friction.

Example III A-blend of 80% Dacron 54 and undrawn polyester fiber of alower softening point was processed on carding equipment to form a battweighing grams per square yard. This batt was unified by passing througha 4-roll textile calender heated to 380 F. at which point the 20% ofundrawn polyester fiber softens and acts as a binder fiber. Theresulting unified sheet was two-thousandths of an inch thick and had atensile strength of 3.18 pounds per inch-wide strip.

Samples of this material were heated at 400 F. for 5 minutes in acirculating air oven, without pressure or dimensional restraint beingapplied. No dimensional changes were observed, and the heat-treatedproduct had a strength of 4.02 pounds per inch-wide strip, an increaseof 25% over the base material.

I have found that the strength-increasing effects of this invention arenot confined to fibrous arrays consisting entirely of fibers on thesurface of which crystalline growth can be induced. but that such fiberscan be mixed or diluted with what may be termed insensitive fibers, torealize strength increases of lesser degree, depending on the amount ofdilution. For example, a series of intimate blends of 1% inch 3 denierrayon was made with 1% inch 3 denier Dacron 54 fiber, in which theDacron fiber was present to the extent of 25 and These blends werecarded and then needle-loomed under identical conditions to form aseries of needled batts of varying fiber composition. All batts,together with a batt of Dacron 54 fiber of comparable weight, were thenheattreated for 3 minutes at 400 F.

The strength of the treated 100% Dacron 54 batt was 20 times thestrength of the base needled material; the 75% Dacron-25% rayon blendwas increased 15 fold; the 50% Dacron-50% rayon blend was between 9 and10 fold; and-the 25 Dacron-75% rayon blend was between 4 and 5 fold.This indicates that the strength increase is probably a straight-linefunction of the Dacron 54 content.

It also exemplifies a method of substantially doubling the strength ofneedled rayon batts by the inclusion of about 10% of Dacron 54 fiberfollowed by appropriate heat treatment according to this invention. Thisstrength increase, moreover, is effect-ed without shrinking, fusing, orotherwise changing the fiber dimensions, and without the addition ofbonding agents or other foreign material to the fibrous array.

As starting fibers for the practice of this invention, I

prefer to use synthetic polymeric fibers which do not exhibit strongretractive forces when heated. Although strength increases are shownboth by low-temperature long-dwell tests and by high-temperatureshort-dwell tests, the latter method, :as explained above, is generallya more desirable commercial process since it may be made continuous witha set of carding machines or other fiberarranging devices, a needleloom, and an oven.

By suitable choice of fibers considered commercially as non-retractible,and by operating below the fusing or sticking point of the fibers, Iarrive at strength increases which do not involve undesirable shrinkageof the array, so that there is no appreciable change in density due tocompacting, nor is there any substantial stiffening or change in hand ofthe product. Thus the invention is especially adapted to the productionof soft, porous, conformable, dimensionally stable fibrous arrays,characterized by a strength which is not attainable by any other processwithout sacrificing at least some of the desirable properties.

Having thus described my invention, what I claim is:

1. A non-woven array consisting at least in part of essentiallyunretracted and unfused synthetic fibers, said fibers having rough andirregular surfaces characterized by the adherent inclusion on saidsurfaces of irregularly distributed fiat prismatic crystals of fibersubstance of a lower degree of polymerization than the main body of saidfiber substance.

2. The product according to claim 1 wherein the array of fibers is abatt of intermingled fibers.

3. The product according to claim 1 wherein the array of fibers is aneedled batt.

4. The product according to claim 3 wherein the array is composed atleast in part of polyethylene terephthalate fibers.

References Cited by the Examiner UNITED STATES PATENTS 2,736,946 3/56Stanton et a1 16l180 2,889,611 6/59 Bedell 2876 2,915,806 12/59 Grant28-81 2,960,752 11/60 Sonnino 28-76 3,023,483 3/ 62 Steiner 28- -813,057,038 10/62 Soehngen 161173 3,096,563 7/ 63 Messinger 161-469 XREARL M. BERGERT, Primary Examiner DONALD W. PARKER, Examiner.

1. A NON-WOVEN ARRAY CONSISTING OF AT LEAST IN PART OF ESSENTIALLYUNRETARACTED AND UNFUSED SYNTHETIC FIBERS, SAID FIBERS HAVING ROUGH ANDIRREGULAR SURFACES CHARACTERIZED BY THE ADHERENT INCLUSION ON SAIDSURFACES OF IRREGULARLY DISTRIBUTED FLAT PRISMATIC CRYSTALS OF FIBERSUBSTANCE OF A LOWER DEGREE OF POLYMERIZATON THAN THE MAIN BODY OF SAIDFIBER SUBSTANCE.